The use of construction members which are fire resistant and fire-propagation delaying or retardant has gained increasing importance in recent years, especially for high-rise construction. A particularly important structural member of this type is a beam (girder) or column which has a steel structural shape, profile or section (e.g. an I-beam or T-beam) whose compartments are filled with concrete. One type of support, which has no concrete on the outside of a flange, so that at least the outer surface of one flange is exposed, is described in German Patent document DE-OS 28 29 864. The concrete filling the spaces between inner surfaces of the flanges and the web of the metal section, preferably an iron or steel section, is secured to the girder or column by attachment means welded on the inside cross piece or web of the metal section or girder, in order to avoid a loosening of the concrete both at room temperature and at temperatures reached in a fire.
The metal cross section, concrete cross section, and reinforcement cross section contribute load supporting characteristics to the girder or column in accordance with their proportions of the total section and their temperature dependent strengths.
In a fire, with rising temperature there is a continuous load redistribution from the metal or steel section to the concrete section because of the softening of the flange, which constitutes the largest part of the metal section contributing to strength.
Since the reinforcement elements in the concrete have not been positioned optimally heretofore, to counteract the detrimental effects which might occur in a fire, the commonly owned Luxembourg application LU 84 772 teaches the provision of at least one additional steel structural shape, profile or section in the concrete, which is attached to the web of the main section, whose outer flanges are not provided with a concrete cover (see also copending applications Ser. No. 603,509 filed 24 Apr. 1984 and Ser. No. 639,375 of 9 Aug. 1984). When a part of the total section comprising a combination of those sections is mounted in a thermally protected zone, a high bearing strength even under fire conditions, is guaranteed.
Patent document LU 84 966 further describes a kind of combined section, in which at least one iron bar attached to the web of the section is mounted in the concrete.
It is also known to make beam-support joints by means of plates mounted on the above mentioned support, wherein the plates are welded to the flanges of the support or to the web of the support. These measures can be taken on the working sides of these structures. In this structure the beam must be welded to those plates by its web or else mounted by screws on it. Subsequently the recesses or cavities required for the mounting work are filled with concrete. These joints are work sensitive and presuppose a completely concrete covered beam and support, which can be unacceptable for a given class of resistance to fire.
It is also known to mount a beam on a support by brackets. The brackets can be attached to the flange of the support. It is also possible however to weld a plate to the two opposite sides of the flange and to attach a bracket for mounting the beam to this plate. The lower flange of the beam will lie on this structure on the bracket (and eventually be welded there), while both sides of the upper flange of the beam can be welded to the support flange.
In another embodiment, the so called cover plate bracket joint, the beam ends are each provided with a cover plate. The lower flange (and a side of the cover plate) contacts on the bracket, while in the vicinity of the upper beam flange, the cover plate is screwed to the support flange. Here longitudinal or transverse holes in the flange and/or the cover plate are provided. The holes or recesses required for the screwing operations are also subsequently filled with concrete.